Theories concerning microtine population dynamics have relied increasingly on assumptions about social structure, but there has been no underlying theory for social biology itself. Among microtine species, patterns of sex-specific territoriality, the best-studied aspect of their social organization, are diverse, setting the scene for the fragmented nature of the contemporary approaches to this issue. Here I propose a set of hypotheses to predict under what conditions females and males should defend territories. The argument has three main components. 1. Microtines have a low-quality diet and should tend to be food-limited. Females invest a large portion of their energy in reproductive physiology, and their reproductive success should be limited by their ability to acquire food and convert it into weaned offspring. Males invest much less energy in reproductive effort and do not participate in parental care; thus, their reproductive success should be limited by access to females. Therefore, territoriality in females should be food-based, and in males it should be female-based. 2. There are three main dietary categories for microtines: fruit-seed-fungus, forbs, and grasses. Although more work is needed to describe the spatial and temporal patterns of availability for these foods, a first approximation is possible. In general, fruits and seeds (in forest habitat) and forbs (in meadows) should be patchily distributed, relatively sparse, and poorly renewable. Since patchy distribution of a resource increases its defensibility, and since sparseness and poor renewability increase the costs of allowing exploitation by intruders, females relying on these two diets should be territorial. On the other hand, grass (in meadows) is evenly distributed (relative to forbs), abundant, and highly renewable. Thus, females of those species that rely largely on grasses should be nonterritorial. 3. When females are territorial they tend to be hyperdispersed, and when females are nonterritorial they should be clumped in distribution. Since evenly spaced females are difficult to defend, male territoriality should not occur in species showing female territoriality. Conversely, when females are nonterritorial, males should be territorial. I assess this argument by using published information. The relation between diet and female territoriality holds up well, with the caution that diet and behavior are seldom analyzed in the same population at the same time. Both factors are variable and their concurrent study necessary. The relation between female territoriality and male territoriality also holds up well, with the exception of Microtus ochrogaster, in which male-female pairs are territorial. Finally, several correlative and experimental approaches are suggested as tests of my predictions. A better understanding of the factors affecting microtine territoriality, and spacing behavior in general, should allow us to evaluate the suppositions of the many theories of how these populations are regulated.
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